1. Field of Invention
The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a perforating system adapted to withstand high wellbore pressure.
2. Description of Prior Art
Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore. The casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
Perforating systems typically comprise one or more perforating guns strung together, these strings of guns can sometimes surpass a thousand feet of perforating length. In FIG. 1 an example of a perforating system 4 is shown. For the sake of clarity, the system 4 depicted comprises a single perforating gun 6 instead of a multitude of guns. The gun 6 is shown disposed within a wellbore 1 on a wireline 5. The perforating system 4 as shown also includes a service truck 7 on the surface 9, where in addition to providing a raising and lowering means, the wireline 5 also provides communication and control connectivity between the truck 7 and the perforating gun 6. The wireline 5 is threaded through pulleys 3 supported above the wellbore 1. As is known, derricks, slips and other similar systems may be used in lieu of a surface truck for inserting and retrieving the perforating system into and from a wellbore. Moreover, perforating systems may also be disposed into a wellbore via tubing, drill pipe, slick line, coiled tubing, to mention a few.
Included with the perforating gun 6 are shaped charges 8 that typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing. When the high explosive is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge 8 at very high velocity in a pattern called a “jet” 12. The jet 12 perforates the casing and the cement and creates a perforation 10 that extends into the surrounding formation 2.
With reference to FIG. 2 to a side partial sectional view of a perforating gun 6 is shown. The perforating gun 6 an annular gun tube 16 in which the shaped charges 8 are arranged in a phased pattern. The gun tube 16 is coaxially disposed within an annular gun body 14. On an end of the perforating gun 6 is an end cap 20 shown threadingly attached to the gun body 14. On the end of the perforating gun 6 opposite the end cap 20 is a lower sub 22 also threadingly attached to the gun body 14. The lower sub 22 includes a chamber shown having an electrical cord 24 attached to a detonator 26. As is known, an associated firing head (not shown) can emit an electrical signal that transferred through the wire and to the detonator 26 for igniting a detonating cord 28 that in turn detonates the shaped charge 8.
Provided between the gun body 14 and gun tube 16 is an annulus 18. The pressure in the annulus 18 is substantially at the atmospheric pressure where the perforating gun 6 is assembled—which is generally about 0 pounds per square inch gauge (psig). However, shaped charge 8 detonation often takes place deep within a well bore, where the ambient pressure can often exceed 5,000 psig. As such, a large pressure difference can exist across the gun body 14 wall thereby requiring an enhanced strength walls as well as rigorous sealing requirements in a perforating gun 6.